The proposed research will examine the contribution of the superior colliculus and the midbrain reticular formation to flash evoked macropotentials recorded from both the visual cortex and the midbrain reticular formation of rats. Procaine hydrochloride will be injected into the superior colliculus and the midbrain reticular formation by micro-injection through indwelling cannulas. Since procaine temporarily blocks both the conduction and generation of action potentials, it will effectively create a "reversible lesion." Flash evoked potentials will then be recorded from both the visual cortex and the midbrain reticular formation. A comparison of evoked potentials recorded following procaine administration with evoked potentials recorded following saline administration will form the basis of the analysis. In the case of evoked potentials recorded from the visual cortex, the results of the proposed study will help determine the often suggested, but little studied, contribution of these mesencephalic structures to the secondary complex of evoked potential components. The results of the proposed research will also help determine the poorly understood origin of flash evoked potentials recorded from the midbrain reticular formation. More generally, the knowledge gained from the proposed research will aid in the interpretation of alterations in flash evoked potential components as a result of aging, disease, drug administration, etc.